Automatic telephone system



y 1936- s. PETERSON AUTOMATIC TELEPHONE SYSTEM Filed Dec. 31,

, INVENTOR.

SETH PETERSON ATTORNEY.

Patented May 12, 1936 PATENT OFFICE AUTOMATIC TELEPHONE SYSTEM Seth Peterson, Oak Park, Ill., assignor, by mesne assignments, to Associated Electric Laboratories, Inc., Chicago, 111.,

Delaware a corporation of Application December 31, 1934, Serial No. 759,823

I 7 Claims.

The present invention relates in' general to automatic telephone systems, and more in particular to the busy test arrangements in such systems by means of which a calling subscriber is prevented from obtaining access to 9. called line if such line is busy. The object of the invention, broadly stated, is to render the busy test or guarding arrangements of anautomatictelephone system more reliable and to make it impossible to defeat their purpose by unauthorized procedure on the part of the subscribers.

The invention is illustrated herein as applied to the well known Strowger step by step system, although it may conceivably be useful in other systems. In the Strowger system a connection is set up responsive to a dialling operation by the calling subscriber, which causes one or more selector switches and a connector switch to extend the calling line to the called line. The connector is provided with means including testing circuits which determine if the called line is idle or not and which prevent the completion of the connection if the called line is engaged. These operations are performed satisfactorily so long as calls are made in the regular way.

It has been discovered, however, that a particular type of connector which is widely used is capable of being'operated in such a manner, by a calling party who is familiar with the neces-- sary routine, as to establish connection with a busy called line, notwithstanding that the test and guarding circuits are in perfect working condition. The situation has become known to a few subscribers in operating exchanges, giving rise to complaints, and had it not been corrected, would no doubt have developed into a serious condition. More specifically, therefore, the object of the invention is to overcome the defect above referred to, the desired result being accomplished by altering the connector circuit in such a way that the operations which formerly were successful in causing the connector to cut in on a busy line are now entirely ineffectual.

The invention will be described more in detail hereinafter, reference being made to the accompanying drawing, which shows by means of the usual circuit diagrams sufiicient of the apparatus of an automatic telephone system to enable the invention to be explained and understood.

Referring to the drawing, the reference character C indicates a connector switch of well known type such as has been hereinbefore mentioned, which is capable of connecting with a busy line if operated in a particular way. This connector may be assumed to be one of a group of connectors having access to one hundred subscribers lines, conforming to the customary practice in the Strowger system.

One of, the lines to which the connector C has access is shown at the right of the drawing and comprises conductors Ill and H leading to the subscribers station A. The line is multipled in the banks of the connectors such as C so as to provide for incoming calls. Outgoing calls are made by means of the rotary line switch RL, which has access to selectors. These features are all well understood and need not be explained in detail.

The reference character C indicates a connector switch which in general is similar to the connector C, but has its circuit altered according to the invention so as to accomplish the object thereof as previously pointed out. For con- ;venience in explaining the invention it has been assumed that the connector C is in the same group with connector ,C and therefore also has access to the line of station A.

s The normal operation of the connector C is well known, but will be briefly described nevertheless, preliminary to explaining its forced or abnormal operation and the manner in which such operation is prevented in the case of connector C.

Assuming that the connector C has been seized by a selector, the calling line will be extended to trunk conductors l3 and I5, which results in the energization of the line relay 3, followed by the energization of the release relay 4. The latterv grounds the release trunk conductor [4 so as to provide a holding circuit for the preceding switches.

The calling party now dials the tens digit in the number, which causes the line relay 3 to deenergize momentarily a number of times, sending impulses to the vertical magnet ID. This steps up the wipers l6, I1, and I8 until they stand opposite the level in which the called line is terminated. The slow relay 5 is in series with the vertical magnet and cooperates with the offnormal springs ON to transfer the impulse cir cult ,to the rotary magnet II at the end of the vertical operation.

The calling subscriber may now dial the last or units digit of the called number. The line relay operates as before, but now sends a series of impulses tothe rotary magnet II. Accordingly,.the wipers l6, l1, and I8 are rotated step by step until they come to rest on the called line terminals. It will be assumed that the called line is the line of station A and that the wipers are positioned on contacts 20, 2|, and 22.

The slow relay 8 is connectedin parallel with the rotary magnet and is in operated position during the rotary movement. At its armature '24, relay 8 connects the'test Wiper I! with the test relay 1. The test relay opens the magnet circuit at its armature 2'! and since the relay is liable to be operated during the rotary movement test contact 2|.

by the test wiper I! engaging grounded contacts of busy lines the relay 8 is provided with an armature 26 for the purpose of maintaining the rotary magnet circuit closed.

Assuming now that the rotary movement has just ceased, with the connector wipers positioned on the called line, the relay 8 will remain operated for an instant and hold the test wiper I! in connection with the test relay 1. If the called line is busy there will be a ground potential on test contact 2| and relay 1 will energize. When relay 8 falls back a locking circuit is established for relay 1 in an obvious manner. Relay 1 holds open the rotary magnet circuit so as to prevent the connector from being moved off the busy called line by further operation of the dial, and also applies a source of busy tone current to the calling line.

Assuming now that the called line is idle when connected with, the test relay 1 will not pull up because the test wiper l1 finds no ground on Then when relay 8 falls back a circuit is completed for energizing switching relay 9, extending from the grounded conductor I4, by way of contacts of test relay 1, the upper winding of relay 9, contacts of relay 8, test wiper l1, test contact 2|, the cut-off relay 40 of line switch RL and the stepping magnet 4| of the said line switch to battery. Relay 9 pulls up in series with the cut-off relay, which latter performs the usual well known functions. Relay 9 is slightly slow to operate, to give time for the operation of the cut-off relay. As the relay armature is attracted, the pair of springs represented by armature 29 and its working contact are closed first, thus completing an energizing circuit for the lower locking winding. The two windings then cooperate in fully operating the relay and thus all the springs of the relay are actuated. Armature 30 grounds the test wiper l1, armature 3| opens the rotary magnet circuit, and the upper and lower armatures connect up the line wipers l6 and I8. Ringing current is now projected out over the called line and when the called man answers the ring-cut-off relay 6 is operated. This completes the talking connection.

The release of the connector, whether the called line is busy or idle, is controlled by the release relay 4, which falls back if the line is opened for more than a very short interval, and

'closes a circuit for the release magnet I2. The

magnet circuit is broken at the 01f -normal springs when the switch shaft reaches normal position. The connector is of the release by last party type and therefore the releasing operation is delayed if the called party fails to hang up.

The manner in which the connector 0 can be made to establish connection with a busy line will now be explained. For this purpose it will be assumed that the wanted line is the line of station A and that the telephone number assigned to that station ends with the digits 1 and 2; That is, the number, if a four digit number,

maybe represented by $3012. The calling subscriber will naturally first dial the number in the usual way, but finds the line busy, as denoted would dial the number small. It has to be assumed at this point that the line having the number mall is idle, otherwise the attempt will be unsuccessful. Now, just an instant after the dial returns to normal, the correct interval has to be determined by experiment, the switchhook is momentarily depressed so as to send another impulse. If the operation is timed right the connector will be advanced one step onto the busy line xx12 and will out in, completing the desired connection.

To describe what happens the operation of the connector C may be adverted to, especially the operation of the switching relay 9. When the connector wipers come to rest on the contacts of line .rxll, the line is found idle, as assumed, and consequently the test relay 1 does not energize. When relay 8 falls back, a circuit is completed for switching relay 9 and this relay starts to operate, first closing its locking circuit at 29. Now, if the extra impulse caused by manipulation of the switchhook comes in just about the time contact 29 is closed, or shortly before, it will be efiective to operate the rotary magnet ll, since contact 3| has not yet opened, and the switch wipers will be advanced one step onto the busy line 22x12. At the same time, if contact 29 has closed, there is nothing to prevent the full operation of relay 9, and the desired result of cutting in on the busy line is attained. The critical part of the operation is the correct timing of the extra impulse. If the impulse comes in too soon the circuit of the upper winding of relay 9 will be broken by the operation of relay 8 before the relay 9 has operated far enough to close contact 29, and relay 9 will fall back. Relay 1 will then operate as soon as wiper I? is moved onto the busy test contact by the rotary magnet. On the other hand if the impulse comes in too late, the relay 9 will have operated far enough to open contact 3 l so that the impulse will be ineffective. Thus, if the impulse is too early the subscriber will get the busy line while if too late he will remain connected with line acwll. Some skill is necessary, therefore, and moreover, the connectors vary a little in adjustment which adds somewhat to the difficulty. Nevertheless, it has been found that after a little practice almost any one can get in on a busy line with not more than two or three attempts and frequently on the first attempt.

Attention may now be directed to the connector C, and the wiring changes which have been made for the purpose of defeating the establishment of such unauthorized connections. There are only two changes. The upper armature of relay 4 is not grounded as it is in the case of connector C, but is connected to the test wiper ll over a conductor 32. Also the locking winding of relay 9' is supplied with battery over a conductor 33 extending to rotary magnet H, instead of being connected direct to battery as in the locking winding of relay 9 of connector C.

The normal operation of connector C in calling a busy line is precisely the same as was described in the case of connector C and consequently need not be referred to. Furthermore, the operation of connector C in calling an idle line is exactly the same as that of connector 0, except for the circuits of the switching relay 9. Therefore, the explanation will be started at the point where the switching relay operation comes in. I

Relay 8 having fallen back at the end of the rotary movement, a circuit for the switching relay 9 is completed which extends from grounded conductor M by way of armature 23' and its back contact, upper winding of relay 9, back contact and armature 24, test wiper ll, test contact 2|, cut-01f relay 40, and magnet 4| to battery. Relay 9 operates in series with relay 4!], and first closes its contact 29', whereupon the circuit conditions are somewhat altered. Ground at conductor l4 extends by way of 28, upper winding of relay 9, and 24' to junction point 34, where the circuit divides, one branch extending over the previously traced path by way of relay 40 to battery, and the other branch extending by way of conductor 32, 29', the locking Winding of relay 9, conductor 33, and rotary magnet II to battery. The two windings of relay 9 assist each other in the circuits above traced and the relay is fully operated, with results the same as those described in the case of connector C. The magnet l I, it will be understood, is of relatively low resistance as compared to the locking winding of relay 9 and consequently may be neglected in so far as its effect on the relay operation is concerned. Also, the current through the relay is, of course, far too low to operate the magnet. It may be stated that calculations show the circuit conditions after the closure of contact 29 of relay 9' to be slightly more favorable for the rapid completion of the operation of the relay than they are in the ordinary connector circuit such as the circuit of connector C. However, this fact is not especially relied on, although it is believed to be of some slight advantage.

It will be assumed now that a calling subscriber attempts to establish connection with the line of station A while the line is busy, following the hereinbefore explained procedure by which such operation may be accomplished with the connector C. Using the connector C, the connection cannot be obtained, for reasons which will be pointed out.

If the connector is dialled on. to the preceding adjacent line $3711, and if that line is idle, a circuit for the switching relay 9 will be closed, as explained, and this relay will start to operate, closing its locking contact 29. As described in the case of connector C, the extra impulse must be delivered almost or quite coincidentally with the closure of the locking contact or the operation has no chance of success at all, and the same conditions, of course, obtain in the case of connector C. That is, the usual features of the circuit will defeat the attempt unless the impulse is correctly timed. Assuming, therefore, that the impulse is timed correctly, it will come in just about at the instant contact 29' closes. We then have two effective circuits for relay 9'. The first circuit is the one extending from grounded conductor l4 through the upper winding of relay 9' and via the test wiper to battery at the rotary line switch. The other circuit extends from the grounded impulse circuit (the impulse is now being delivered to the rotary magnet), by way of conductor 33, locking winding of relay 9', contact 29', upper contacts of relay 4, conductor 32, junction point 34, and by way of test wiper H to battery at the rotary line switch. In the fore going circuits the upper and lower windings of relay 9' are in opposition and the relay is therefore instantly deenergized by the differential action of the windings. The deenergization of relay 9', of course, permits the extra impulse to always effect the advance of the switch wipers onto the busy line, but at the same time it insures the energization of test relay 1, which pulls up as soon as test wiper l1 moves over onto busy test contact 2|. Thus the attempt to gain access to the busy called line is defeated, and the calling party is given a busy signal.

It will be seen that the desired results are secured without adding any relays, springs, or other parts to the connectors, which makes the circuit advantageous not only from the standpoint of economy in manufacturing new equipment, but also because of the facility with which the equipment in existing exchanges may be altered to incorporate the invention.

Having described my invention, what is considered to be new and is desired to be protected by Letters Patent will be set forth in the following claims.

What is claimed is:

1. In a connector switch, a switching relay, a stepping magnet for setting the connector on a called line, means for energizing said switching relay, and means for deenergizing the said switching relay if the stepping magnet is reoperated at a time when the switching relay has not completed its operation.

2. In a connector switch, a double wound switching relay, means for energizing the two windings of said relay cumulatively to operate the relay when an idle line is connected with, and means for energizing said windings differentially if an attempt is made to step the switch while the switching relay is operating.

3. In a connector switch, a switching relay, a stepping magnet, means for energizing said switching relay when an idle called line is connected with, a locking winding for said relay, a holding circuit for maintaining the relay operated, said circuit including the said locking winding and said stepping magnet in series, and a circuit including the said locking winding and said magnet in parallel which is established if an attempt is made to operate said magnet while the said switching relay is operating responsive to said energizing means, the effect of said last circuit being to oppose the said energizing means and cause the relay to deenergize.

4. In a connector switch, a switching relay having two windings, means for setting the connector on a called line, a circuit for energizing one winding of said relay in series with the cutoif relay of the called line if the line is idle, a holding circuit for the other winding, and a circuit for energizing said other winding in series with said cut-off relay if an attempt is made to reset the connector while the switching relay is operating, said windings being in opposition when said last circuit is established and being effective to deenergize the relay.

5. In a connector switch, a double wound switching relay, a circuit for one winding including the test wiper of the switch, and a circuit for the other winding including the said first winding but excluding the said test wiper.

6. In a connector switch, a double wound switching relay, a stepping magnet, a circuit for one winding which excludes the other winding and the magnet, and a circuit which includes both windings and the said magnet in series.

I. In a connector switch, a test wiper, a release relay, a switching relay, a locking conductor for the switching relay, and contacts on said release relay for connecting said locking conductor to said test wiper over a path which excludes said switching relay.

SETH PETERSON. 

